Devices are known for measuring fluid flow using both static and dynamic pressure measurements. Dynamic pressure is the component of fluid pressure that represents fluid kinetic energy (i.e., is responsive to fluid motion), while static pressure represents only hydrostatic effects. Dynamic pressure results from the force of a fluid impinging on a surface whereas hydrostatic pressure measures only static pressure. Hydrostatic pressure exists regardless of whether the fluid is flowing, whereas dynamic pressure exists only when fluid is flowing. Traditional differential pressure (“DP”) devices measure the hydrostatic pressure drop across a structure inserted into the flow stream specifically designed to produce a substantial pressure drop (e.g., an orifice plate) in order to measure flow rate. Hydrostatic pressure differentials measured are smaller than dynamic pressure signals resulting in very limited dynamic flow measurement range.
U.S. Pat. No. 6,865,957 (“the '957 patent”). The '957 patent teaches a mass flow measurement by subtracting two dynamic pressure measurements. With reference to FIG. 4 of the '957 patent, a first dynamic pressure measurement P1 is obtained at the apex of a conically-shaped end-piece; a second dynamic pressure measurement P2 is obtained at a different location (i.e., different radial location) from a pressure port located within the “cylindrical portion” of the body. According to the '957 patent, solely the difference between these two dynamic pressure signals determines fluid mass flow rate. For example, the '957 patent states at column 4, lines 29-31, “FIG. 5 is a schematic flow diagram depicting the process employed in determining mass flow with the device of the present invention” and FIG. 5 explicitly illustrates that P1−P2 determines mass flow. At col. 6, lines 51-54, “The difference between pressures P1 and P2 are then determined either manually or automatically and, through the use of tables or graphs determine the mass flow of fluid through device 10.”